The present invention generally related to apparatus for measuring viscosities of oils and more particularly to an improved apparatus for accurately predicting engine cold cranking characteristics, i.e. engine viscosity of a crank case oil.
Generally, the problems associated with accurately predicting cranking characteristics of engine oils in an engine operating at low temperatures, for example below 0.degree. F., are well known. It is also well known that the viscosity of these oils influence the force (i.e. torque required to crank an engine and the engine cranking speed,) and that the viscosity of any oil is temperature dependent. The viscosity of crank case oil influences the ease with which an engine will start at low temperatures.
Those associated with the automotive industry, such an engine manufacturers, oil processors, automobile owners and the like, are extremely interested in being able to determine the viscosity of an engine oil without having to evaluate the oil under controlled conditions in an actual engine. Cranking characteristics of certain categories of non-additive oils such as solvent neutral mineral oils known as "straight mineral" oils can be fairly accurately predicted without actual cranking by using the viscosity at the temperature of interest, for example 0.degree. to 20.degree. F. The viscosity of these oils is obtained by extrapolating low shear viscosities obtained at 100.degree. and 210.degree. F. on the ASTM viscosity temperature chart. However, this procedure is not totally satisfactory for those oils characterized as non-Newtonian which comprise a large majority of the crank case oils presently used. Typical examples of the latter oils are the solvent neutral oils containing viscosity index improvers such as polymethacrylates, often copolymers, polybutenes and styrene polyesters. Use of the extrapolated viscosity technique which can be employed to predict engine cranking characteristics of the "straight mineral" oils are not applicable to non-Newtonian oils.
Heretofore, an instrument which has been used in determining the viscosity of those oils which are not subject to the extrapolated viscosity technique is disclosed in U.S. Pat. No. 3,350,922 assigned to the assignee of the present invention. In that apparatus, a pair of flats are employed on the rotor for permitting air bubbles to escape as the oil is introduced into the annular test space. These flats tend to create a situation of non-uniform shear rates which cannot be tolerated in testing of non-Newtonian oils at very low shear rates, i.e. about 0.05 to about 2 sec..sup.-1. The non-uniformity in shear rate has only a very negligible effect on the accuracy of viscosity measurements obtained at high shear rates used in this apparatus, commonly known as a cold cranking simulator. However, most oils at temperatures below 0.degree. F. are highly non-Newtonian, this means that the viscosity of these oils is highly dependent on the shear rate. Wax separation is a common cause of this low temperature Non-Newtonian behavior which is frequently complicated by the polymer-wax interaction. Oils with wax separation show a very steep increase in viscosity with a decrease in shear rate in the low shear rate region. In latter situations the flats provided on the rotor produce extremely large inaccuracies in the measurements obtained because of much lower shear rates inside the flats.
An example of this might be employing a test oil PRO-10 having a viscosity of 885 poises at a shear rate of 2 sec.sup.-1 and a temperature of -30.degree. F. At the same temperature, and at a shear rate of 1 sec.sup.-1 the same oil will have a measured viscosity of 1390 poises. Still at the same temperature, but at an even lower shear rate of 0.5 sec.sup.-1, the viscosity of the same oil reaches 2620 poises. It also has been found that the inaccuracy because of the non-uniform rates introduced by the flats is not a constant error and varies with shear rate and temperature of the non-Newtonian oil undergoing testing. The low-shear-rate viscosity at low temperature is extremely important in its relation to the engine failure due to air binding at the oil pump inlet. Air binding is the result of insufficient oil movement, i.e. insufficient flow at an extremely low shear rate, in the sump for the oil to reach the oil pump inlet because of excessively high oil viscosity at the low shear rate. When air binding occurs, the oil pump picks up only air and sends no oil out to lubricate and engine failure follows quickly.